1. Field of the Invention
The present invention relates generally to magnetic resonance imaging, and in particular, to a method, apparatus, and article of manufacture for high resolution two-dimensional tomographic magnetic resonance imaging using magnetic probes.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Magnetic resonance imaging [1][2] (MRI) has advanced at a rapid pace since initial proposals and demonstrations in 1973 [3][4], with applications in medical imaging attracting the most attention [5][6]. Advances in magnetic resonance microscopy have also been significant [8], recently reaching two-dimensional (2D) imaging resolution of 1 μm [9]. Improvements in conventional inductive methods of magnetic resonance detection [10][11] and application of imaging gradients have generally been used to achieve such advances. Although the possibilities of angstrom-scale resolution were pondered in the early days of MRI [4][12], the ultimate goal of achieving atomic resolution has remained elusive.
In 1991, an alternative method of applying the imaging gradients and detecting magnetic resonance, magnetic resonance force microscopy (MRFM)[13] was proposed, with the ultimate goal of single spin sensitivity and three-dimensional (3D) imaging capability. The technique relies on the atomic scale imaging gradients from the microscopic magnetic particle mounted on a micromachined mechanical cantilever for the appropriate detection sensitivity required for 3D single spin imaging [14]. Successful MRFM demonstrations were reported for the cases of electron spin [15] nuclear spin [16], and ferromagnetic [17] resonance systems. MRFM research has benefited from the low temperature implementations of the instrument [18], and rapid advances in the fabrication techniques for incorporating smaller magnetic particles [19][20], and more sensitive mechanical resonators [21]. However, reported MRFM imaging resolution of ˜1 μm [22][23] remains at the level of inductive detection in conventional MRI.
A complementary atomic resolution magnetic resonance imaging method [24] may be used to significantly relax the challenging technical requirements of single spin detection by imaging discrete ordered crystal lattice planes where many spins coherently contribute to the magnetic resonance signal. Such an approach closely resembles the initial magnetic resonance imaging proposal [4][12] in which linear magnetic field gradients are used to selectively excite magnetic resonance in different atomic lattice planes and produce “diffraction”-like effects. However, the approach differs from the initial magnetic resonance imaging proposal by introducing nonlinear magnetic fields and field gradients from a ferromagnetic sphere to achieve atomic resolution magnetic resonance diffraction. By using various sample-detector coupling mechanisms [25], it may be shown that the realization of the long desired atomic resolution magnetic resonance diffraction of crystals may be within reach of available experimental techniques.
However, the prior art lacks the ability to conduct two-dimensional magnetic resonance microscopy of noncrystalline structures at an atomic resolution.